Liquid coolant systems can play a major role in the performance and safety of the device being cooled. For example, the coolant system can be especially important for power sources, such as internal combustion engines, fuel cells, and nuclear reactors, and for semiconductor chips such as microprocessors. In an internal combustion engine, the coolant system can be used to maintain engine operating temperature sufficiently high that the combustion system operates at or near peak efficiency while preventing engine temperatures so high that engine components are damaged. Likewise, the coolant system of a nuclear reactor is important in avoiding reactor damage and catastrophic reactor failure. In some fuel cells, coolant is carefully controlled at elevated temperatures but boiling of coolant should be avoided. Similarly, it is well known that semiconductor chip performance can be enhanced, or performance decay can be avoided, by keeping the semiconductor chip in a specified temperature range. Coolant systems in HVAC systems, refrigerators, and other refrigeration or cooling devices can also be controlled to improve system efficiency and to avoid damage. Other areas where coolant control can be useful are liquid cooled machine tools and casting and manufacturing processes.
FIG. 1 graphically illustrates energy transfer from a heated surface, such as a surface in a coolant circuit having a surface temperature T, to a liquid, such as a coolant, in contact with the surface, the coolant having a saturation temperature Tsat. For relatively low heat flux q″, excess temperature or superheat (Ts−Tsat) is less than T1 and heat transfer from the surface to the coolant is by way of convective heat transfer. The convective heat transfer can be natural or forced, depending on whether the coolant is flowing across the surface. As heat flux q″ increases, nucleation occurs and superheat rises to a range between T1 and T2. In this state, discrete bubbles are formed at the surface and so-called “nucleate boiling heat transfer” occurs. As heat flux q″ increases even more, the coolant begins to approach boiling crisis. Discrete bubbles are no longer formed but instead columns or slugs of vapor are formed and merging bubbles occur. Although heat transfer in this state is a form of nucleate boiling heat transfer, it is often referred to as “slug boiling” or “aggressive boiling” heat transfer. Superheat is between T2 and T3 in this state. Further increasing heat flux q″ drives the liquid toward the peak of the graph of FIG. 1, at which point heat flux q″ reaches a local maximum and superheat reaches T3. This point is referred to as “critical heat flux” (CHF), “departure from nucleate boiling” (DNB), “boiling crisis”, “onset of film boiling” or “burnout”. Once the heat flux q″ reaches critical heat flux, any further increase will cause the coolant to depart from nucleate boiling and jump to a state of film boiling heat transfer in which a film of vapor is formed across the surface, thus preventing the formation and departure of bubbles. During film boiling heat transfer, heat flux increases only a small amount, if at all, but superheat rapidly increases by several hundred degrees to T4 as the coolant transitions along the horizontal dashed line in FIG. 1.
To enhance heat transfer from a heated surface in a coolant circuit to the coolant medium, it is known to use liquid coolant in a nucleate boiling state rather than relying on only natural convective heat transfer since, as shown in FIG. 1, higher flux q″ (as compared with convective heat transfer) is possible with a relatively low superheat and thus a relatively low surface temperature Ts. As well known, nucleate boiling occurs when bubbles form at the heated surface due to pockets of vapor trapped in cavities or other imperfections in the surface. As the bubbles depart the surface, lower temperature coolant spaced from the surface is drawn down to the surface, which decreases the surface temperature Ts. However, as mentioned above, as heat flux increases, the coolant transitions into a slug boiling state and approaches critical heat flux or departure from nucleate boiling. In internal combustion engine, for example, coolant that is in a nucleate boiling state, and particularly a slug boiling state, can undesirably depart from nucleate boiling during periods of high engine loading or after the a hot shut down when coolant is no longer circulating. Similarly, in semiconductor chip cooling for example, prolonged operation of the semiconductor chips, such a microprocessors, at high loading or a rapid shut down of computing system could result in film boiling. Thus, the coolant can transition instantly to a film boiling heat transfer regime in which the surface temperature Ts can rapidly rise to an extremely high temperature at which damage or even catastrophic system failure can occur.
Thus, while the benefits of nucleate boiling can be harnessed to improve coolant system performance, coolant system designers have traditionally designed systems to avoid nucleate boiling heat transfer in order to avoid the damaging transitions to film boiling heat transfer. However, with regard to internal combustion engines for example, the increasing stringency of engine emissions regulations is expected to result in more demanding heat rejection requirements as higher engine operating temperatures and cooled exhaust gas recirculation, for example, are used to reduce engine emissions. Increased heat transfer demands are also anticipated in connection with semiconductor chip cooling as processing speed and chip density continue to increase. Thus, the heat transfer benefits of nucleate boiling are expected to become more attractive.
Efforts have been made to harness the energy transfer benefits of nucleate boiling in coolant systems. For example, U.S. Pat. No. 4,768,484 to Scarselletta discloses a coolant system in which coolant is maintained in a state of nucleate boiling at a selected location in the coolant passages of an engine. A coolant pump is controlled to adjust the static pressure of the flowing coolant at the selected location so that nucleate boiling occurs at the selected location. Temperature and pressure sensors supply signals to a microprocessor that uses look-up tables to predict whether, based on the sensed temperature and pressure, the coolant is in a nucleate boiling state and then controls the speed of the coolant pump to maintain the coolant at the selected location in a state of nucleate boiling. However, systems such as described in U.S. Pat. No. 4,768,484 have shortcomings because they require multiple sensors and an electronic controller with extensive look-up table capabilities, an expensive and delicate pressure sensor, and knowledge of the coolant's physical properties. Such systems are also subject to error associated with the randomness and unpredictability of departure from nucleate boiling resulting from the random nature of surface cavities and imperfections. Thus, to make such systems practical, high cost and low durability components are likely required together with control algorithms that permit only low efficiency, early-stage discrete nucleate boiling so that errors in the boiling state calculations (e.g. from wrong coolant properties, sensor error) do not accidentally allow a damaging transition to film boiling.
In the context of semiconductor chip cooling, it is known to utilize nucleate boiling heat transfer improve semiconductor chip performance. However, because undesirable transitions to damaging film boiling are possible unless the coolant system is carefully controlled, damage to the semiconductor chip is a concern. Damaging the semiconductor chip, especially a microprocessor, can be especially troublesome for critical high-performance computer systems, since microprocessor damage can result in costly system down-time and potentially loss of valuable data.
Accordingly, a need exists for a liquid and/or coolant system that can be used to maintain liquid/coolant in a desired state (e.g. discrete nucleate boiling) and thus avoid transitions to undesired states (e.g. slug boiling, film boiling), but which is relatively simple, robust, and provides accurate quantitative and/or qualitative information about the state of the liquid/coolant.